Not true.

So, unlike in Muirchu's account of the conversion of Ireland, no one found Patrick so threatening as to warrant a call to arms over Christianity. There was never a recorded act of violence between Christian and pagan, nor was there a single martyrdom in Ireland over the conversion to Christianity (Hopkin 21). Although Patrick began the process of introducing the Irish to Christianity, it does not appear that he had nearly the phenomenal success that later writers would attribute to him. In fact, Patrick himself died in obscurity. Far from being the arrogant miracle-worker who made disbelievers pay for their skepticism, the historical Patrick "was not remembered as an enormously successful missionary—because he was not enormously successful. At the time of his death Ireland was still predominantly pagan, aggressively pagan" (Thompson 158).

http://www.strangehorizons.com/2009/20090727/da_silva-a.shtml

I've written about this exact same topic too many times on this subreddit, so I'll paste one of my earlier responses to the same question:

It's total BS. Ireland's Christianization was incredibly peaceful compared to conversions elsewhere (there were no martyrs in Ireland's transition to Christianity) and actually preserved many aspects of pagan belief and superstition, rather than rooting them out.

Christianity was actually introduced to Ireland even before St Patrick through Roman Britain, and evidently spread amongst the most politically powerless segments of Irish society; women and slaves. Indeed, Patrick wasn't even Ireland's first evangelist; a man named Palladius was sent to "the Irish believing in Christ" in 431 AD. Christianity was adopted incredibly quickly in Ireland not because it was successful at rooting out and destroying pagan structures, but because it Christianized pagan beliefs and traditions. Healing wells and springs associated with deities were now associated with local saints, and Irish warriors now called upon Jesus, Saint Patrick or Saint Columba for victory in battle rather than their pagan gods. Christianity's success can be linked to its widespread adoption by the Irish aristocracy, whose druids seem to have quickly become Christian clergy, and whose warrior-aristocrats were likely convinced by its superiority as a form of military magic.

Irish pagans weren't philosophically attached to their religion; they just used whatever magical forms of manipulation could work best for them. The belief in magical manipulation was central to the belief structures of all pre-Christian Europeans and survived in popular belief until the Modern period. When Christianity justified those forms of magic by attributing them to Jesus or God, there was no real barrier for an Irish pagan to become an Irish Christian - there was less of a cataclysmic break from the past than a blended transition where elements of old structures of belief survived because the clergy couldn't risk alienating the majority of the population.

As well, Christianity with its clear doctrine and corpus of literature, filled with apparently magical events, could justly assert itself as a fundamentally more powerful way of attaining magical power. Drawing on Old Testament God's ability to destroy cities and flood the earth, Christianity was evidently more powerful than paganism whose gods were more localized and had more diffused power. Why would you worship a bunch of gods with different, limited powers when you could worship one supreme god who could give you all your benefits at once? With Christianity, one could harness a single god's power for prowess in battle, fertility for their crops, protection from weather or plague etc. instead of having to supplicate a whole bunch of different deities and spirits. Not only did Christianity simplify the process of gaining magical powers, but it also had a whole corpus of events in the Bible that could be used as proof of the new Gods' power, while Ireland's pre-Christian belief system had no doctrine or dogma and thus lacked Christianity's persuasive power.

Pagans were never subjugated in Ireland Christianization. Their beliefs were actually adopted by the Christian church (in fact, almost everything we know from pre-Christian belief in Ireland comes from Christian texts!) and they willingly adopted it because of its perceived magical power.

Wait, the Hagiography of Saint Patrick has him as a British man who was captured and enslaved by Irish slavers and forcibly taken to Ireland. How on earth could anyone turn that in to a story of Saint Patrick being an oppressor of the Irish?

'Help help. I am being oppressed by my slave!'

It sounds like a Monty Python sketch.

As to the snakes legend, Ireland never had snakes, whilst mainland Britain did, this is because there was a land bridge between Britain and the European continent during the last ice-age, whilst the Island of Ireland was isolated, so the snakes legend is literally referencing actual snakes and seeking to explain why Ireland has no native species of them.

Oh and Irish Christian missionaries were very significant in the conversion of the Anglo-Saxons to Christianity, the idea that Christianity spread only from Britain to Ireland is totally inaccurate revisionism.

There are no contemporaneous sources that state that Saint Patrick banishing the snakes from Ireland is a metaphor for the Pagans being driven out. However there are also no contemporaneous sources that declare Ireland became a Pagan free zone during his mission either. Obviously the “Vita sancti Patricii” written by Muirchu in the seventh century puts this premise forward but other evidence would appear to speak against Saint Patrick’s success and to Muirchu’s revisionism.

For example according to the “Annals of Tigernach”, Diarmait mac Cerbaill was proclaimed High King of Ireland in around 558 after celebrating the Feast of Tara. This festival took place to symbolise the marriage of the prospective king to the land and ended only when a giant stone bellowed its acceptance of the candidate. Not particularly Christian despite it being almost a century after the accepted date for Saint Patrick’s death. A century later again sees the Bechbretha mention that Congal Cáech lost the High Kingship due to no longer being “whole” having being stung by a bee. This pre-Christian way of thinking could be traced back to the mythic tales of Nuada Silver Hand.

There are also many, many examples of the various Irish Christian saints in the years after Patrick’s death converting local Pagan kings. The most famous example would be Saint Brigid’s cloak. Of course many of these stories are probably allegorical but having survived 1500 years in the telling there must be some truth to them and for the most part they “occur” post Saint Patrick.

However, the most damning evidence against any notion that a campaign of violent conversion occurred against Pagans and Druids during Saint Patrick’s tenure has to be the “Bretha Crólige” and the “Uraicecht Becc”. These are legal texts dating from around the 7th or 8th Century. To be brief, Irish law at that time ranked everyone from king to slave and set a certain value to each grade of people. This ranking was used for everything such as determining how much a murderer should pay their victim’s family or if a person incapacitated another how much they must pay their victim in sick maintenance. Literally compensation culture.

Anyway the “Bretha Crólige” ranks a Druid as a bóaire or freeman farmer in regards to what should be paid in sick maintenance. However tellingly the “Uraicecht Becc” ranks a Druid amongst the doer nemed or professional class alongside such exalted company as blacksmiths. This dependant rank means that the Druid would have as a patron a lord capable of affording him. The fact that Druids remains in a relatively high social status on the statute books so to speak, 150 years after Patrick finished evangelising, makes it ridiculous to believe he was responsible for the Druidic tradition’s destruction.

Also all of this conjecture depends on believing that early Christians thought the snake represented Pagans. I am aware of no evidence beyond modern speculation that suggests Irish Druids wore snake tattoos as part of their profession of faith. However In the Gospel of John, 3:14 – 15 Jesus himself uses snakes as a symbol of salvation. Moses tramped the desert with his staff that healed people when it turned into a snake. Which religion then would have the better claim to its espousal?

In all likelihood Paganism died out organically in Ireland due to several factors. The Church maintained a stranglehold on literacy and therefore education. Druids would have found themselves increasingly sidelined as their sacred sites were appropriated for Christian worship and their traditional role of advisers to the local kings was replaced by Priests as evidenced by their inclusion in the Brehon laws several rungs above Druids. In much the same way as the Pontifex Maximus’s title was inherited by the Pope, the role of Druid was taken by the Priest. However it was almost certainly taken with a whimper and not a bang.

I think any conclusion could be best served by Saint Winwaloe who was urged to be kind to a local druid because he “has lost his gods! What sorrow can compare with this sorrow? Once he was a druid; now he mourns a dead religion.”